This invention relates to an adhesive composition useful in the preparation of multilayered laminates. The adhesive is particularly useful in adhering dissimilar materials such as barrier layer to a polyolefin layer for forming durable, moldable articles which resist delamination.
The preparation of rigid or semi-rigid polymeric containers that can be handled and processed similar to metal cans or glass jars has long been sought for economic reasons and for the general improvement of packaging. Therefore, it has been desirable to replace the use of metal or glass with more durable and light packaging materials. Laminates of various polymeric materials are preferred to package goods because they provide flexibility in choosing what properties each portion of the container will have. For example, the inner surface of a container should provide a surface which will not contaminate the packaged goods while securely containing them. The interior of the container should prevent the passage of gases and vapor, i.e., comprise a barrier layer, and the exterior of the container should provide structural support for the container and resist breakage. Finally the entire laminate should be uniformly moldable such that the various properties contributed by the polymeric layers are consistent throughout the container.
Problems associated with laminates generally, be they sheet or film structures, involve adhering the various polymeric layers together such that they will not delaminate when molded or processed. Typically, the conditions which potential laminate containers must undergo aggravate the tendency of dissimilar layers to separate. For instance, the molding process can put great stress on the interface of the polymeric layers due to different tensile and elongation properties of the polymers. Further, the processing requirements, in the processing of food containers, can subject the laminate to retort conditions in order to cook or sterilize the packaged material. It is therefore critical that an excellent adhesive composition be employed to compensate for the difference in the physical and chemical properties of the dissimilar polymeric layers. Various adhesive compositions capable of being molded and processed have been suggested but better systems capable of undergoing even greater stress are still being sought.
One such attempt is disclosed in U.S. Pat. No. 3,673,050 which provides for a five-layer rigid or semi-rigid sheet comprising an inner barrier layer having a glue layer on both sides which is adhered to rigid polyolefin outer layers. The adhesive layer is composed of a copolymer of ethylene and vinyl acetate. Another attempt at forming a multilayered structure which can be coextruded as a film or sheet is disclosed in U.S. Pat. No. 3,908,070. There a nylon resin layer is adhesively bonded to a polyolefin layer to form a barrier to gas and water vapor. The adhesive layer is a copolymer of ethylene and monocarboxylic acids such as acrylic acid or methacrylic acid.
The present invention concerns an improved adhesive composition for adhering laminates comprising a grafted polypropylene, a glycol and a poly-2-oxazoline which provides excellent adhesion and resists delamination under the severe conditions which can occur during the molding of the laminate and processing of the packaged goods. This particular combination has been found to provide both an adhesive and compatibilizing layer between dissimilar polymeric layers.